436 



The midgut is lined by what appears to be a very delicate 

 cuticle. Tlie epithelial cells lining it are much larger than 

 those occurring elsewhere in the intestine. The great accumu- 

 lation of food within the midgut soon stretches them (fig. 10), 

 and already at the end of the first instar they are becoming 

 flattened; they measure at this stage about 68/x in length 

 (and breadth), 11/x in thickness. The cell cytoplasm is vacuo- 

 lated and granular. The nucleus is large (17/x) and is very 

 heavily granular. 



At the base of many of these cells there is frequently 

 to be observed a much smaller cell, spindle-shaped, about 

 17/x in length, 4/x in height (fig. 10). Each has a large clear 

 nucleus and a distinct karyosome, and represents an- un- 

 differentiated non-functioning cell, which will become active 

 during the defaecation period, and will form the great endo- 

 dermal intestine of the early pupa. The anterior portion of 

 this, as will be described later, will disintegrate, while the 

 hinder will persist as the stomach of the imago. It is, there- 

 fore, possible to speak of these cells when they occur in the 

 posterior region of the intestine~as imayinal stoinach cells; 

 the more anterior ones cannot be thus described. I shall 

 refer to them later as replacing cells. 



External to the epithelium is a rough network of longi- 

 tudinally and circularly disposed muscle fibres. 



The rectum (figs. 143, 185) is a prominent ectodermal 

 ingrowth through the anus — a true proctodaeum, but does 

 not yet open into the midgut. It is a rounded tube, lined 

 by chitin, with a columnar, or, in places, cubical epithelium. 

 Surrounding it is a layer of circularly disposed, as yet un- 

 striated, muscle fibres. The epithelial and muscular cells still 

 retain the usual undifferentiated appearance of clear cyto- 

 plasm and large ''vesicular" nulcei. 



The cells at the anterior end of the proctodaeal invagina- 

 tion fit tightly against the rear of the midgut, and form a 

 layer several cells in thickness (fig. 143). The cells here are 

 slightly smaller than elsewhere and constitute the imaginal 

 disc of the hindgut, from which the small intestine and rectum 

 of the imago will later develop. 



The salivary glands. These consist each of a large sac- 

 like structure — the secreting portion, and a duct of medium 

 length, the two ducts uniting before opening into the mouth. 

 In the first instar the common duct is rather flat and strap- 

 like; along its middle passes a prominent canal, lined by a 

 chitinous spiral intima, which is shed and reformed at each 

 moult. Further behind, the duct becomes circular in section, 

 and is composed of very small cells with the usual ''first- 

 instar" appearance, viz., large clear ''vesicular" nucleus, with 



